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In electrical consumption measurement activities, there is a tendency to underestimate the choice of current transformer.
This can cause inconvenience both in installation by the operator, with time losses that can come from the difficulty of installation or going back to the company to retrieve the right Current Transformers, to the introduction of measurement errors that can affect the energy measurement from our energy meter.
We have seen that there are two macro categories of current transformers: one dedicated to ex-novo implementations, so closed-loop current transformers, and the other open-loop type that is more oriented to energy monitoring retrofit implementations, where we have to go and install these CTs on existing switchboards.
But what types of Openable Current Transformers are available in the market? And what is the right choice to make?
We must consider, in addition to the application, two main factors: one, is the amount of current we want to measure, and two, is the cross-sectional area of the cable to be handled..
We know that there is a relationship between these two factors: the more the current increases, the more the cross section should increase.
To approach these measuring instruments, we have to make a choice that depends on the installation distance from the instrument to the Amperometric Transformers, the amount of current, and the performance we want to achieve. strong
Typically the secondaries of current transformers are expressed in current (1 A or 5 A) which are known values). Lately, voltage-expressed secondaries (milliVolt output) are also gaining popularity, which are giving significant advantages from an installation point of view. This is because management with mV signal is much simpler than management with Current signal.
Having a smaller amount of energy, it is possible to install current transformers with mV output at considerable distances from our energy meter, as it is possible to lengthen the cable according to our needs, and the cable cross section is reduced. Therefore, there will be less installation costs and fewer variables to consider . In addition, the cost of such an amperometric transformer for low currents is lower than those with a 1 A or 5 A secondary.
With much higher amperages, you no longer find cables to carry the current but bars. At this point, the opening TA must also have suitable mechanical characteristics to be installed in these applications. With this type of openable CTs, you can go to read currents from 100 A up to 6000 A with rectangular holes 20x30mm, 60x80mm, 80x120mm, 80x160mm
We have cables and bars. We have amperes ranging from 5/30A up to 6000A.
But what happens when we have variable currents and do not have suitable current transformers for installation?
Recently, measurement probes that are called Rogowski coils have been emerging. What are they? They are current transformers wound no longer on a ferromagnetic material, but are air-wound conductors. The special feature of these probes is the fact that, precisely because of their design feature, they are not subject to saturation. Cores do not saturate, so they do not have a full scale. It is therefore possible to go from a few tens of amperes up to thousands of amperes with a single solution.
This type of probe is not supported by all measuring instruments , as they introduce phase shifts in the input waveform. So they must be handled with special care.
However, we have the possibility by choosing the suitable instrumentation to have the solutions to go and solve any kind of installation.